1. Field of the Invention
This invention relates to electron trapping materials that are capable of storing radiographic images and a process for making and using such material.
2. Description of the Prior Art
Until recently, X-ray radiography was based entirely on photochemistry. Images of different parts of the body were obtained by exposing photographic film and then developing the film in conventional ways. The difficulty has always been that the silver halides used in film chemistry are not especially sensitive to X-rays. Thus, the patient was exposed to a radiation level which was determined solely by the level required to properly expose the film. The radiation level was eventually reduced with the introduction of fluorescent intensifying screens, which helped to reduce the X-ray exposure of the patient.
Since major advances have been made in electronic data handling, it has become possible to form images that no longer involve wet chemistry. Digital processing offers many advantages, such as high resolution, potentially greater sensitivity, reusability and easier storage. The sensitivity, which is controlled by the solid state target film, holds the key to reduced dosage.
Recently, Fuji using the technology disclosed by Kotera in U.S. Pat. Nos. 4,239,968 and 4,261,854 started to employ a photostimulable phosphor placed inside an X-ray cassette. After exposure of the phosphor the image is read in a "darkroom" by a laser scanner. The image appears in the form of light emission which can then be sensed and stored digitally or can be printed as a hard copy. Through the manipulation of the digital data base, contrast and other features of the image can be altered in conventional ways.
Gasiot, et al in U.S. Pat. No. 4,517,463 discloses a real time radiation imaging apparatus and method. Gasiot's real time device required rapid release of stored luminescent energy. Gasiot et al suggested phosphors such as calcium, strontium, magnesium and barium sulfides doped with europium and samarium compounds for providing rapid release of light. Barium sulfide doped with cerium and samarium was found acceptable to Gasiot, et al as was barium fluoro-chloride.
Human exposure to X-rays can be reduced somewhat through the use of these techniques. However, truly significant reduction in human exposure levels was not achieved because sufficiently sensitive phosphors with intense light output were not available.
The particular type of phosphor required belongs to a unique family of election trapping optical materials. In order to define this family of materials, it is useful to review its history, particularly since sometimes confusion exists over terminology. It is important to begin with the term luminescence, the ability of certain solids to emit light under different conditions.
Luminescence is a long known phenomenon of nature reaching back very far in history. Recorded observations reach back to the last century. Seeback and Becquerel observed momentary visible afterglow in certain materials. In 1889, Klatt and Lenard also observed some effects with infrared. During this time period, words like "phosphor" and "luminescence" appeared. In 1904, Dahms distinguished between "stimulation" and "quenching"; meaning inducing or stopping afterglow. Much of the later work is associated with Lenard, who received the Nobel Prize in 1905 in physics for cathode ray emission. He studied different phosphors until at least 1918. Later work can be found by Urbach in 1926 through 1934. These early scientists basically observed very small luminescent effects.
In 1941, a program was instituted by the National Defense Committee for development of light emitting phosphors. The work started at the University of Rochester, and other laboratories became involved; however, the projects ended with World War II. The following technical papers were published on this work between 1946 and 1949:
B. O'Brien, "Development of Infrared Phosphors", J. Opt. Soc. of Am., vol. 36, July 1946, p. 369;
F. Urbach, et al., "On Infrared Sensitive Phosphors", J. Opt. Soc. of Am., vol. 36, July 1946, p. 372;
G. Fonda, "Preparation and Characteristics of Zinc Sulfide Phosphors Sensitive to Infra-Red", J. Opt. Soc. of Am, vol. 36, July 1946, p. 382;
A. L. Smith, "The Preparation of Strontium Selenide and its Properties as a Base Material for Phosphors Stimulated by Infra-Red", Journal of the Am. Chem. Soc., vol. 69, 1947, p. 1725;
K. Butler, "Emission Spectra of Silicate Phosphors with Manganese Activation". Journal of the Electrochemical Society, vol. 93, No. 5, 1948, p. 143; and
"Preparation and Characteristics of Solid Luminescent Materials", Editors: G. R. Fonda and F. Seitz. John Wiley & Sons, Inc., New York, 1948.
These papers provide an early story on the materials studied. As decades went by, the effects were forgotten by most physicists. Only work in the field of cathodoluminescence for screens of cathode ray tubes and fluorescent lamps continued with any focus.
Thus, the field of luminescence is broad and refers to the ability of certain substances or materials to emit light when driven by an external energy source. When the driving energy source is light, the proper term is photoluminescence.
The most interesting class of materials are those which upon excitation by radiation can store electrons in "traps" for varying lengths of time as discussed by J. L. Summerdijk and A. Bril in "Visible Luminescence . . . Under I R Excitation", International Conference on Luminescense, Leningrad, August 1972, p. 86. In the case of deep traps, trapped electrons can be released at a later time by photons having an energy similar to the depth of the trap. Thermal discharging is negligible in the case of deep traps. Under these circumstances, it appears that information corresponding to an excitation radiation can be stored for later use. The information can be reconstructed in the form of visible light emission, activated by infrared. These materials are now called electron trapping optical materials.
The fundamentals of electron trapping material are the following: A host crystal is a wide bandgap semiconductor (II-VI) compound, normally without any special value. These crystals, however, can be doped heavily with impurities to produce new energy levels and bands. Impurities from the lanthanide (rare earth) series are some of the elements that can be accommodated in the lattice to form a "communication" band and a trapping level. The new communication band provides an energy band in which the untrapped electrons can interact. The trapping level at yet lower energies represents non-communicating sites.
Materials that display latent luminescent activity often include one or more types of sites where electrons may be trapped in an energized state. Upon application of suitable wavelengths of energizing radiation, such as visible light or X-rays, such sites become filled with electrons. The electrons are raised to an energized state via the communication band from which transitions, such as absorption and recombination, may take place. Upon removal of the energizing radiation, the electrons may be trapped at an energy level higher than their original ground state or may drop back to their original ground state. The number of electrons that become trapped is very much dependent upon the composition of the photoluminescent material and the dopants used therein.
If the trapping level is sufficiently below the level of the communication band, the electrons in them will be isolated from each other, will remain trapped for a long period of time, and will be unaffected by normal ambient temperatures. Indeed, if the depth of the trap is sufficient, the electrons will remain trapped almost indefinitely unless they are activated by specific light energies, or thermal energy much higher than room temperature.
The electrons will remain trapped until light or other radiation is applied to provide sufficient energy to again raise them to the level of the communication band, where a transition may take place in the form of recombination allowing the electrons to escape from the trap and release photons of visible light. The material must be such that room temperature thermal energy is insufficient to allow any significant portion of trapped electrons to escape from their traps. As used herein, "optical energy" shall include visible light, infrared light, ultraviolet light, X-ray, gamma radiation, beta and alpha particles unless otherwise noted, "photoluminescent material" is a material that exhibits the above characteristics.
Although various photoluminescent materials have heretofore been known, the properties have often been less than desirable. For example, photoluminescent materials have been used for locating infrared beams by outputting visible light upon placement of the material within an infrared beam, but such previous photoluminescent materials are not sensitive enough an emit relatively low levels of light. In the same manner, phosphors used in X-ray radiography required such high levels of X-ray radiation that most of the expected benefits of reduced exposure to humans were not realized.
The ratio of input energy to energy of light output in such materials is often very high. That is, a large amount of energy must be put into the material to provide a modest output of optical energy. The development by the applicant of photoluminescent materials that avoid or minimize the disadvantages discussed above opened up numerous practical applications for such materials.
The patents of Kotera and Gasiot et al cited above point to two separate advantages of their phosphor materials. Kotera claims his barium fluorohalide phosphors used in X-ray information storage have a sensitivity improvement factor of over a 1000 compared to Sm and Eu doped SrS or CaS based phosphors, suggested by Gasiot et al for their readout speed advantage over the barium fluorohalide phosphors of Kotera.